1. The Field of the Invention
The invention generally relates to the field of fiber optic communications. More specifically, the invention relates to gain flattening filters for equalizing gain from an amplifier in an optical system.
2. The Relevant Technology
In the field of data transmission, one method of efficiently transporting data is through the use of fiber-optics. Digital data is propagated through a fiber-optic cable using light emitting diodes or lasers. Light signals allow for high transmission rates and high bandwidth capabilities. Also, light signals are resistant to electro-magnetic interferences that would otherwise interfere with electrical signals. Light signals are more secure because they do not allow portions of the signal to escape from the fiber-optic cable as can occur with electronic signals in wire-based systems. Light signals also can be conducted over greater distances without the signal loss typically associated with electronic signals on wire-based systems.
While signal loss in a fiber-optic cable may be less than signal loss in wire-based systems, there is nonetheless some signal loss as light signals are transmitted over fiber-optic networks. Optical amplifiers are used to compensate for the signal loss. Two common optical amplifiers are Raman amplifiers and Erbium Doped Fiber Amplifiers (EDFAs). Both of these amplifiers use characteristics of doped fiber-optic cables to amplify light signals.
The amplifier pumps light onto the fiber-optic cable. The pumped light is at a different frequency than the light signal that is to be amplified. As the light signal and pumped light travel along the fiber-optic cable, energy from the pumped light is transferred to the light signal. Optical amplifiers use optical pumps, i.e. laser sources, to generate the pumped light.
In some fiber-optic applications, the light signals being transmitted may include different wavelengths of light. Each wavelength may be referred to as a channel. For example, the C-band might be used to transmit 40 different channels or wavelengths along the 1530 to 1562 nm bandwidth. In a variety of optical applications, it is desirable to amplify each channel with about the same optical gain. However, the optical gain of an optical gain medium, such as the doped fiber-optic cables, depends upon wavelength. In other words, optical amplifiers like Raman amplifiers and EDFAs, do not provide the same amount of optical gain to each channel in the light signals and some wavelength channels experience greater amplification than other channels. Consequently, a single gain medium does not usually function as a high gain medium having substantially uniform optical gain over an extended wavelength range.
Conventional approaches to providing uniform optical gain over an extended wavelength range typically have more components than desired, require significant numbers of optical interconnects resulting in insertion losses, and typically cost more than desired. Illustratively, EDFAs are widely used to amplify optical signals to compensate for transmission losses and insertion losses caused, for example by interconnection of components. The gain characteristics of EDFAs are a strong function of optical wavelength. Therefore, to achieve substantially uniform optical gain over an extended wavelength range, an additional gain equalization filter (GEF) he is needed in addition to the EDFA. In a single stage optical amplifier, GEFs are commonly placed after the final stage of the amplifier. For multi-stage amplifiers, GEFs are sometimes placed between amplifier stages. Each GEF introduces an additional component cost, component size, and requires appropriate packaging to permit it to be optically coupled to other components. Further, physically coupling components together results in some degree of insertion loss for each physical connection.
One type of GEF is based on a thin film having a wavelength sensitive transmission curve G(f). Once an incoming beam I(f) passes through the filter, the out-coming beam O(f) can be described as:O(f)=G(f)*I(f)  (1).
The requirement of GEF transmission dynamics, or the amount of the variation in gain among the frequencies being equalized, depends on the application. In some cases, a wide dynamic range (e.g., more than 10 dB) is required to ensure that the game of each channel is equalized. This can make the GEF filter difficult to make and expensive.